Henry et al, that is a picture that even goes back to the 1940`s,and perpetuated in AP129,but it is still wrong. The lift vector `L`,always should point to the centre axis of the `barrel`. You can do it as per the diagram,but can end up `tent-pegging`,as many have......,if at low-level.....
Actually if you look at the Transall video,the pull-up is started wings-level,and then at about 35-40 deg. N-U,aileron is fully applied,as applying full aileron at max `G` is not a good idea in such a large,long-span aircraft,but the rest looks good,also having the benefit of airbrakes reduces the speed/G/ roll in the latter part of the manoeuvre..
There is/was a heavy discussion on the `Flypast` website,of a photo supposedly showing a Dakota doing a barrel roll,but no-one actually produced the photo....Syc..